Objective To determine the relationship between serum ferritin and malnutrition in newly assessed patients at a paediatric eating disorders clinic.

Design This was a prospectively assessed clinical cohort study.

Setting Intake assessment clinic of a tertiary eating disorders service for children and adolescents.

Methods Clinical, anthropometric and laboratory features of children and adolescents were systematically measured. The relationship of serum ferritin to other clinical, anthropometric and laboratory measures was determined using linear regression.

Results A total of 121 female patients aged 9.5–17.6 years were included, with body mass index (BMI) z score −5.7 to 1.9 (median −1.3). Using multiple regression, serum ferritin was inversely associated with BMI z score (regression coefficient (β)=−0.234, 95% CI −0.413 to −0.055) and serum insulin-like growth factor 1 (IGF-1) (β=−0.476, 95% CI −0.884 to −0.068) and positively associated with alanine aminotransferase (β=0.357, 95% CI 0.055 to 0.659, controlling for age, pubertal stage and serum iron).

Conclusions In malnourished adolescents with eating disorders increased serum ferritin is associated with lower BMI z score and serum IGF-1.